1. Field of the Invention
The present invention relates to an active vibration damping system including a vibration damping device having a fluid chamber filled with a non-compressible fluid and mounted on a subject member whose vibration is damped by the damping system, and a control device for controlling the pressure of the fluid within the fluid chamber, so as to positively damp or isolate the input vibration. In particular, the present invention relates to such an active vibration damping system suitably used as an engine mount or any other type of vibration damper for an automotive vehicle.
2. Discussion of the Related Art
For damping or isolating a vibration (including a noise induced by the vibration) of a subject member such as a body of an automotive vehicle, there have been employed:
a vibration damping device such as an engine mount or a suspension bushing, which is interposed between the subject member and a vibration source such as a power unit so as to connect these two members in a vibration damping fashion for eliminating or reducing a vibration transmitted from the vibration source to the subject member; and a vibration damper such as a dynamic damper which is fixed to the subject member for absorbing or reducing the vibration of the subject member. As one type of such a vibration damper, there are known active vibration damping systems as disclosed in laid-open publication No. 61-191543 of Japanese Utility Model application, Japanese Patent No. 2510914 and Japanese Patent No. 2510915, which have been developed so as to meet a recent requirement for improved vibration damping characteristics. Such an active vibration damping system includes: a vibration damping device having a fluid chamber filled with a non-compressive fluid and partially defined by an elastic body which is elastically deformable upon application of an input vibrational load from the subject member to the vibration damping device, the fluid chamber being further defined by an oscillating plate which is displaced or oscillated by a suitable drive means; and a control device for applying an electric drive signal to drive means for controlling the oscillation of the oscillating plate. The electric drive signal corresponds to the vibration to be damped by the damping system. The oscillation of the oscillating plate causes a periodic change of the pressure of the fluid within the fluid chamber, so as to positively isolate or damp the vibration of the subject member.
For obtaining an excellent vibration damping effect of the vibration damping device constructed as described above, the waveform of an oscillating force to be applied to the oscillating plate or the waveform of a pressure change of the fluid in the fluid chamber is required to meet or suit the particular characteristics of the vibration of the subject member as much as possible. In this respect, there has been proposed an engine mount as disclosed in JP-A 8-72561 and JP-A 9-42374, which is interposed between an internal combustion engine (vibration source) and a body (subject member) of a vehicle. In this engine mount, the control device generates the electric drive signal in the form of a sine wave drive current whose period, amplitude and phase correspond to those of the vibration. The generated drive signal is applied to the drive means such as an electromagnetic drive means or actuator to thereby oscillate the oscillating plate.
However, the control device adapted to generate the sine wave drive current whose waveform corresponds to the waveform of the vibration of the subject member tends to be complicated, inevitably resulting in an increase in the manufacturing cost of the damping device. Further, the complicated control device tends to suffer from generation of a high-frequency noise superimposed on the electric drive signal. More specifically described, the sine wave drive current which has a predetermined relationship with the characteristics of the vibration to be damped is preferably obtained according to an analog control or a pulse duration (width) modulation (PWM) control, for example. According to the analog control, there is initially obtained an analog base voltage signal having a sine waveform which corresponds to that of the vibration of the subject member. The sine wave base voltage signal is modified in the terms of its phase and amplitude (gain) by an analog processing circuit, to thereby obtain the desired sine wave drive current. According to the PWM control, on the other hand, there is initially obtained a digital base voltage pulse signal, which is then subjected to pulse width modulation according to or depending upon the waveform of the vibration of the subject member. The thus obtained digital base voltage pulse signal is applied to the drive means via an H-bridge circuit having switching elements such as transistor. The H-bridge circuit is adapted to control the application of the base voltage signal so as to obtain the desired sine wave drive current to be applied to the drive means. In the former case, i.e., in the case of the analog control, however, a very complicated electric circuit is required for generating the analog base voltage signal having a sine waveform, and for adjusting the phase and the amplitude (gain) of the base voltage signal, inevitably resulting in an increase of the manufacturing cost. On the other hand, the PWM control requires a carrier wave having a considerably high frequency, a central processing unit (CPU) having a large processing capacity e.g., 16-32 bits, for high-speed arithmetic operations to process the base voltage pulse signal at a high frequency, and a memory having a relatively large storage capacity for storing complicated control programs. Thus, the PWM control also inevitably suffers from an increase in the cost of manufacture.
Further, the active vibration damping system as described above is required to a relatively large oscillating force for oscillating the oscillating plate so as to provide an excellent active vibration damping effect. When the vibration of the subject member has a relatively large amplitude, the required oscillating force is accordingly large, requiring a large-sized drive means for oscillating the oscillating plate, resulting in an increase in the size and weight of the vibration damping device, and an increase in the required electric power consumption.